Tool and method for welding to IC frames

ABSTRACT

A device for forming an encapsulated end of a wire bonded to a metal surface. The device includes a clamp being an end surface of a bar is pressed against a surface of a p[ate. The end surface has a ridge formed around its edge. so that when a clamping force is applied to the bar against the plate, the metal sheet clamped between the plate surface and end surface of the bar, the clamping force is concentrated at an interface between the metal surface and ridge. When encapsulant is injected into the contact region, the ridge prevents contaminating material from migranting over interface between bar and plate where the welding step is to be performed. The bar is then withdrawn leaving a tunned through which the wire is positioned with the end of the wire in contact bondable to the clean surfave of metal. The wire is positioned in the tunnel and bonded to the metal surface.

FIELD OF THE INVENTION

[0001] This invention relates to manufacturing integrated circuits andparticularly to a tool and method for butt welding a wire to anintegrated circuit frame.

BACKGROUND AND INFORMATION DISCLOSURE

[0002] Fabrication of semiconductor devices includes two areas ofoperations: forming electrical contacts between wires and terminals ofcomponents, and packaging, i.e., encasing the device in a plastic orceramic enclosure to provide mechanical support and protection from theenvironment.

[0003] Contacts between a wire or pin and a pad (terminal) generallyfall into two categories: permanent and temporary.

[0004] In permanent applications, the requirement is to secure an end ofa wire to a pad. The pad is the surface of a lead frame or the terminalof a semiconductor die. Methods used to permanently bond the end of thewire to such terminations include soldering and ultrasonic welding.

[0005] Contacts of a temporary nature include contacts of conductors toterminals of a test circuit where contact is maintained only during abrief test procedure. In this application, the test wire is commonlysupported in a manner wherein the wire maintains a resilient(springlike) force against the terminal of the device under test.

[0006] Numerous techniques for forming permanent and compressive bondshave been disclosed in the Patent literature. U.S. Pat. No. 3,460,481discloses a technique whereby the wire severing operation comprisesmoving the needle with holding pressure sufficient to frictionallyengage the wire and insufficient to deform the wire away from the bondarea.

[0007] U.S. Pat. No. 3,73,481 to Lins et al discloses forming pin-likegold pedestal structures atop terminal portions of an n integratedcircuit device by thermocompressing gold spheres and shaping the sphereswith a heated vacuum holder.

[0008] U.S. Pat. No. 5,095,187 to Gliga discloses a wire-bondingtechnique involving a combination of heat, pressure and vibration. Thispatent discusses weakening or severing the wire by localized applicationof heat and how the severing operation may result in a broadened portionon the severed end of the wire

[0009] U.S. Pat. No. 6,062,459 to Sabyeying discloses a wirebond clampfor holding fingers of lead frames against a heater block. The clampincludes a base plate having an array of apertures for the lead wires.

[0010] U.S. Pat. No. 6,336,269 to Eldridge et al discloses a flexibleelongate member (such as a gold wire) mounted on a contact area and isconfigured to have a springable shape, such as a wire stem having atleast one bend.

[0011] Packaging is an important part of semiconductor technologywherein the object of the package is to confer mechanical stability andprotection from the environment on a circuit comprising a plurality ofelectrical connections between electronic components.

[0012] Ceramic packages are required where long life of the encapsulateddevice is required. Plastic packaging of the circuit is practiced wherecost is an important consideration with some degree of mechanicalstability and protection from the environment.

[0013] Intensive research into packaging techniques has been directedtoward the development of polymer composites that have improved thermalresistance, lower coefficients of thermal expansion, reduced moistureabsorption characteristics. More recently efforts have been directedtoward the development of three-dimensional packages. Numerous patentsare directed toward these efforts.

[0014] For example, U.S. Pat. No. 5,985,693 to Leedy is directed towardfabrication of integrated circuits from flexible membranes formed ofvery thin low stress dielectric materials and semiconductor materials.

[0015] U.S. Pat. No. 6,147,335 to Von Arx discloses various componentsof thermal systems molded within a polymer sheath. The use of transferand compression molding allows for the use of thermoset polymerscontaining very high levels of reinforcement fillers.

[0016] In a related configuration, it is required to form electricalconnection of the ends of wires to respective fingers of a lead frame.Attempts to form such contacts involve forming a permanent bond of theend of a wire or pin to the broad surface of the finger. Encasing thecontact in a plastic after the contact is formed is difficult to carryout because of the mechanical forces exerted on the contact during thesubsequent molding process.

[0017] Ultrasonic bonding the tip of a wire or pin to the broad surfaceof a lead of a lead frame requires that the surface of the lead bescrupulously clean in order to obtain a strong bond.

SUMMARY OF THE INVENTION

[0018] It is an object of this invention to provide a tool and processfor forming a permanent bond of the tip of one or more wires or pins tothe surface of a metal sheet.

[0019] It is a further object of the invention that the contact(s) beencapsulated in a resin medium.

[0020] It is a further object that, in forming the contact to thesurface of the metal to which the tips are bonded the surface of themetal be devoid of contaminating films including oxidized films thatwould diminish the strength of the bond. It is contemplated that aparticular application of the method and tool be to the formation ofwires and pins bonded to the leads of a led frame.

[0021] This invention is directed toward a process which begins withcleaning the surface of the lead frame and then providing a clamp whoseone jaw is a flat plate and whose other jaw is a bar that is preferablyround. The clamping surface of the bar, (i.e., the end of the bar) has arim around its edge.

[0022] When the sheet is clamped between the bar and the plate, acasting medium is forced under high pressure into the cavity thatincludes the end of the bar forced into contact with the plate. Theforce of the rim of the bar is sufficient to deform the surface of themetal and prevent leakage of the encapsulating medium between the plateand the clamping (end) surface of the bar.

[0023] When the clamp (bar and plate} are withdrawn from the casting,there is left a package including the lead frame with attached leadencapsulated in the casting medium. An open column leads from thesurface of the lead to the outer surface of the package.

[0024] In a second step of the process, a wire (or pin) is inserted intothe open end of the column and into contact with the surface of the lead(of the lead frame) on the opposite end of the open column.

[0025] The ultrasonic energy is then applied to the wire so as toultrasonically bond the wire to the frame.

[0026] The open column enclosing the bonded wire is then impregnated ina second encapsulating step so as to seal the wire in position welded tothe lead frame thereby completing the packaging operation. A crucialfeature in the foregoing process is the use of the hold down bar withthe raised rim around the edge that effectively prevents the liquefiedencapsulating medium from contaminating the surface of the lead whichwould otherwise interfere with formation of the bond between the surfaceof the lead and the wire tip.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1a shows a lead frame.

[0028]FIG. 1b shows the encapsulated circuit

[0029]FIG. 2 is a flow chart illustrating the method of the invention.

[0030]FIG. 3 shows details of the clamping surface of the bar.

[0031]FIG. 4 is a perspective view showing deformation of the leadsurface and details of the mold structure.

[0032]FIG. 5 shows the clamp in the encapsulating mold.

[0033]FIG. 6 shows a sectional view of the lead surface without thebenefit of the ridge illustrating contamination by encapsulant of thelead surface.

[0034]FIG. 7 is a sectional view showing the tunnel formed by the barclamp.

[0035]FIG. 8 shows the wire positioned in the tunnel.

[0036]FIG. 9 shows the tunnel backfilled with encapsulant.

DESCRIPTION OF A PREFERRED MODE

[0037] Turning now to a discussion of the drawings, FIG. 1a shows a leadframe 10 (well known in the art) being a metal lamination etched orstamped to form an array of leads 12, each lead 12 having an outer endintegrally joined to a surrounding frame 14. There is also shown anarray of wires 16 (three are shown) each wire 16 bonded at a selectedlocation for the respective wire. The circuit is entirely hermeticallysealed in an encapsulating medium to form a molding 18 as shown in FIG.1b.

[0038] The steps forming the encapsulated circuit of FIG. 1a are listedin the flow chart of FIG. 2 and illustrated in FIGS. 3-6.

[0039] In step 1, the lead frame with leads shown in FIG. 1a is formedby techniques well known in the art. The process includes leaving thesurface of the lead frame 10 in a “clean” condition that is suitable forforming a bond of a wire 16 to the surface of the lead frame 14 usingthe ultrasonic bonding technique.

[0040] In step 2, as shown in the sectional view of FIG. 3, the lead 12of lead frame 10 is clamped in a clamp including a plate 20 (one jaw)and an opposing jaw 22. The opposing jaw 22 is a round bar with one end24 being the clamping surface.

[0041] The clamping surface 24 on the end of the bar 22 has a ridge 26extending around the edge of the clamping surface 24.

[0042]FIG. 4 is a perspective view the surface 28 of the lead 12deformed leaving a circular depression 29 in response to the force ofthe ridge 26 against the surface of the metal.

[0043]FIG. 5 is a sectional view showing that the plate 20 and bar 22forming the clamp is inside the cavity 21 of a mold 23 to be filled withencapsulating medium. The bar is spring loaded (spring not shown)thereby providing a controlled predetermined force of the end of the baragainst the surface of the metal lead when the mold is closed.

[0044] In step 3, an encapsulating medium is injected into the cavity.The encapsulating medium is any one of a number of polymer/ceramicencapsulating media that are well known in the art of injection molding.The ridge 26 of the bar 22 forced against the lead 12 forms an intimatecontact between the ridge on the end face of bar so as to preventencapsulating medium from being forced in between the bar and metalsurface of the lead 12. FIG. 6 shows that, when there is no ridge on theend of the bar) the liquid encapsulating medium is forced between theend surface 24 of the bar and the lead 12.

[0045] In contrast to FIG. 6, the surface of the lead 12 shown in FIG. 5remains clean and is ready for subsequent wire bonding thereto whereasthe surface of the lead shown in FIG. 6 is contaminated by encapsulatingmedium and must be cleaned by additional steps before ultrasonic bondingcan be performed to the surface.

[0046] In step 4, the clamping bar is withdrawn from the plate, leavingthe encapsulated frame in the mold cavity. FIG. 7 is a sectional viewshowing the tunnel 30 left in the encapsulating medium 32 after the bar20 is withdrawn from the plate. The tunnel 30 extends from the (exposed)surface 24 of the metal lead 12 to the outer surface of the molding 32.

[0047] In step 5, as shown in FIG. 8, a wire 16 is inserted into thetunnel 30 wherein one end of the wire is in contact with the surface ofthe lead 12 and the other end of the wire 16 extends out of the open endof the tunnel 30.

[0048] In step 6 either one or a combination of ultrasonic/bondingenergy is applied to weld the tip of the wire to the lead.

[0049] In step 7, the wire is severed leaving an end of the wireextending out of the open end of the tunnel.

[0050] In step 8, as shown in the sectional view of FIG. 9, a secondencapsulating step is performed to fill the tunnel with encapsulate 33and securing the wire in place.

[0051] A major feature of the invention is the specially formed clampingbar with ridge formed on the perimeter of the clamping surface. Thisfeature is very effective in eliminating several expensive steps thatwould otherwise be required to prepare the surface of the lead forbonding thereto.

[0052] Variations and modifications of this invention may becontemplated after reading the specifications and studying the drawings,which are within the scope of the invention. The scope of the inventionincludes not only the process of making the circuit but also the methodof making the bar (clamp) having a ridge on the end. In one embodimentthe bar is a tube. This configuration has the advantage of greatconvenience in being able to frequently sharpen the end of the bar(tube) to maintain the required ridge configuration.

[0053] It is contemplated that a single mold can be used for numerousversions of a circuit within the molded package. Such a mold isillustrated in FIG. 9 where there is shown a plate holding an array ofbores. Selected bores each hold a spring and one end of a bar. The boresare selected according to the required circuit. Each Bore that is notselected for forming contact with a lead contains a plug.

[0054] It should be understood, in the context of the specification thatthe “plate 20 (one jaw) of FIG. 3 is any member of the mold thatprovides a support to the lead and the bar. In this context, the “plate”in one embodiment is an end of an opposing bar or the internal surfce ofa mold or a member of any configuration that provides the requiredbackup support.

[0055] In view of these versions, I therefore wish to define myinvention by the appended claims.

I claim:
 1. A device for forming an encapsulated circuit, said circuitincluding a wire with an end and a metal sheet having a metal surface,said circuit including said wire having an end contacting said metalsurface, said device comprising: a plate having a plate surface; a barhaving an end surface perpendicular to and meeting an elongated sidesurface of said bar, whereby a comer edge of said bar is defined; aridge formed on said end surface around said comer edge of said bar;means for applying a clamping force to force said ridge toward saidplate surface whereby said ridge abuts an area of said metal surfacewhen said metal sheet is clamped between said plate surface and said endsurface providing that said clamping force is concentrated at aninterface between said metal surface and said ridge; an enclosurearranged to form a molding by injecting an encapsulating medium into aregion within said enclosure that includes said interface between saidinterface and said ridge; said molding, after being separated from saidplate and said bar, includes said encapsulating medium containing aportion of said metal sheet and a tunnel through said encapsulatingmedium left after withdrawing said metal bar from said enclosure; saidtunnel extending from one end coincident with said metal surface and atunnel opening on a surface of said molding providing that said wire ispositionable in said tunnel with an end of said wire in contact withsaid metal surface.
 2. The device of claim 1 wherein said clamping forceis sufficiently great to deform said metal surface.
 3. The device ofclaim 1 further comprising means for bonding said end of said wirepositioned in said tunnel to said metal surface;
 4. The device of claim1 further comprising means for backfilling said tunnel withencapsulating medium.
 5. The device of claim 3 further comprising meansfor backfilling said tunnel with encapsulating medium.
 6. The device ofclaim 3 wherein said metal surface is plated with a solder alloy andsaid means for bonding includes means for passing a current through saidwire having an end in contact with said metal surface, said currentbeing sufficient to solder said end of said wire to said metal surface.7. The device of claim 1 wherein said bar is a tube providing that saidridge is an edge of said tube.
 8. The device of claim 2 wherein saidmeans for bonding comprises means for injecting ultra sonic energy intosaid interface.
 9. The device of claim 8 wherein said wire is a rigidmetal pin and said means for injecting ultrasonic energy comprises: asource of ultrasonic energy; means for coupling said source ofultrasonic energy to an end of said pin extending from said tunnel. 10.The device of claim 1 wherein said metal sheet is a lead of a leadframe.
 11. A method for forming an encapsulated circuit, said circuitIncluding a wire with an end making contact at a location on a metalsurface of a metal sheet, said method including the steps in operableorder: providing a mold having a cavity containing a clamp, said clampincluding a plate being one jaw, and a bar having an end clampingsurface being an opposing jaw wherein said bar has a ridge extendingaround an edge of said clamping surface, said bar slidably supported topermit forcing said end clamping surface against said plate; cleaningsaid metal surface sufficiently to permit forming a bond of said end ofsaid wire to said metal surface; clamping the metal sheet between saidjaws with a clamping force at a location where said contact is required;forming an encapsulation by injecting an encapsulating medium into saidcavity wherein said encapsulating medium is prevented from covering saidlocation by intimate contact between said ridge and said metal surface;withdrawing said clamping bar from said mold, leaving said metal sheetencapsulated in a molding formed in said mold cavity, molding having atunnel that extends from said location on said metal surface to asurface of the molding. inserting a wire is inserted into said tunnelwherein one end of said wire is in contact with said metal surface andanother end of said wire extends out of said open end of the tunnel;applying at least one of ultrasonic and heat energy sufficient to weld atip of said wire to said metal surface at said location; severing saidwire leaving an end of said wire extending out of said open end of saidtunnel; filling said tunnel in a second encapsulating step whereby saidwire is secured in place.
 12. The method of claim 11 wherein saidclamping force is sufficient to form a depression at said location insaid metal surface.